1. Field of the Invention
The present invention relates to a four-wheel steering system attached to a vehicle such as an automobile.
2. Description of the Prior Art
The present applicant has filed Japanese Utility Model Application No. 60-172085 as a four-wheel steering system attached to a vehicle such as an automobile. This system comprises an oil pump (oil pressure source) which is interlocked with gears in a rear-wheel differential device of a rear-wheel drive vehicle or a four-wheel drive vehicle, and a hydraulic circuit for supplying the oil pressure generated by the oil pump to a hydraulic actuator for steering rear wheels. An oil pressure switching valve for in-phase steering is interposed midway along the hydraulic circuit. The switching valve receives the oil pressure generated by a power steering device for front wheels as a pilot signal pressure, and controls an oil pressure supplied from the oil pump to the hydraulic actuator and switches oil pressure supply paths in accordance with the pilot signal pressure, thereby steering the rear wheels in the same direction as the front wheels. In this case, a driving gear of the oil pump is meshed with the gear provided in the differential device. When the driving gear of the oil pump is rotated by the gear of the differential device, an oil pressure corresponding to a vehicle velocity is generated by the oil pump. For this reason, if the front wheels are steered while the vehicle is moving at high speed, high-pressure working fluid is supplied from the oil pump, through the oil pressure switching valve, to the hydraulic actuator which then steers the rear wheels in the same direction as the front wheels, thereby improving travel stability of the vehicle. Since, when the vehicle is moving at low speed, the rotational speed of the gears in the differential device is low, the oil pressure generated by the oil pump is also low. As a result, the hydraulic actuator is not actuated by the oil pressure, and is held in a neutral position. Therefore, if, in this situation, the front wheels are steered, the rear wheels are held in the neutral position, i.e. they are not steered.
However, since, according to the system having the above structure, the rear wheels are held in the neutral, i.e. non-steering, position when the vehicle is moving at low speed, the rear wheels then cannot be steered in the opposite direction to the front wheels, in order to reduce the turning radius of the vehicle, and thus cannot facilitate parking, for example.
Another oil pressure source for the antiphase steering may be provided to steer the rear wheels in the opposite direction to the front wheels while the vehicle is moving at low speed, so that an oil pressure is supplied from the oil pressure source for the antiphase steering to the hydraulic actuator, thereby steering the rear wheels in the opposite direction to the front wheels. However, in this case, an oil pressure source for antiphase steering and a control mechanism for controlling this oil pressure source are required, resulting in a complicated structure and high cost.